Process for the manufacture of malleable iron



Patented Dec. 5, 1933 PROCESS FOR THE MANUFACTURE OF MALLEABLE IRONWalter Cotter-ill, Waterbury, Conn.

No Drawing. Application August 25, 1930 v Serial No. 477,804

6 Claims.

My invention consists of a new and useful process in the manufacture ofmalleable iron.

One object of my invention is to shorten the time required formalleableizing cast iron. Another object is to eliminate the necessityfor packing the castings in powdered material.

Still another and important object is to provide a process formanufacturing malleable cast iron which is easy to control and iscapable of 10 accurate regulation, whereby it is possible to obtainresults which are uniform, and in which it is possible to obtain anydesired degree of malleability within generally accepted limits.

Ordinarily, in the production of malleable cast iron, the castingsaresubjected to a prolonged heat treatment, requiring several days. Thecastings are packed during this prolonged heat treatment in powderediron oxide materials, such as hammer scale, magnetite, hematite, etc.,or

inert materials, such as sand. When the heat treatment is completed, thecastings are allowed to 0001 very slowly in the furnace. The efiect ofthis treatment is to convert the combined carbon in the castings tographite, which takes the form of very minute particles, known as tempercarbon, evenly distributed throughout the casting, the amorphousparticles reposing in the interstices of the crystalline ironstructures, which are opened by the high temperature applications.

The heat treatment also eliminates a portion of the original carboncontent.

My invention contemplates a process of converting hard cast iron intomalleable cast iron, in which a gaseous but a substantially non-oxi- 36dizing atmosphere is maintained in the heating receptacle during therange known as the solid solution range, in which the free cementite (FeC) passes into solution.

It further contemplates the admission of 40 oxygen to the apparatusduring any part of the heat treatment, particularly during thev laterstages of the heating, known as the carbon precipitation range. Thepresence of a relatively small quantity of oxygen-not only aids in theformation of the temper carbon, but also eliminates a certain percentageof the carbon, with.

the result that the products obtained are comparable in strength andother properties with the malleable iron produced by prior processes.The time of heating is cut from several days, which were required in theold processes, to a few hours. Since an inert gaseous medium is employedduring the solid solution range, it is possible to employ a higherheatwithout danger 56 of damage to the castings by burning them, for

example, a temperature of 1700 degrees F. to 1800 degrees F. may beemployed instead of a temperature between 1500 degrees F. and 1600degrees F. Furthermore, it is not necessary to use the powdered packingmaterials heretofore 0o employed, and it has been found that the surfaceof the castings are not roughened or pitted,

as is the case at times when the iron is treated in contact with thepowdered material.

In the practice of my invention the iron-castings to be malleableizedare placed in a heating receptacle capable of withstanding the elevatedtemperatures necessary to carry out the process. The receptacle isequipped with means whereby it may be sealed from the atmosphere and isprovided with one or more inlet pipes for the gases and with an exhaustpipe through which the gases may be removed. The pipes are equipped withvalves to control the admission and elimination of gas. After the ironcastings are placed in the heating receptacle, the receptacle is sealedto exclude the atmospheric air, and the atmosphere in the receptacle isreplaced by an inert and substantially non-oxidizing gaseous medium,which is preferably furnished by a separate chamber in which somecarbonaceous or hydrocarbon materiaL'such as gas or oil, is burned. Forthis purpose it is desirable to employ fuel which can be burned asnearly as possible to complete combustion, in which case 8| the productsof combustion contain 79% of nitrogen and 21% of,carbon dioxide. Whilethis optimum condition is not always possible, it is desirable toapproach it as closely as feasible. Small amounts of carbon monoxide,and even oxygen, are not, however, deleterious.

By means of heat from any suitable source, the receptacle is rapidlyraised to a temperature, preferably between 1700 to 1800 degrees F.until substantially all of the graphitic carbon in the I cast iron hasbeen absorbed by the iron. This usually takes approximately from two tofour hours, depending upon the temperature used. The receptacle is thenallowed to drop gradually in temperature until it reaches .approximate-I 1y 1200 degrees F. This stage is what is known as the carbonprecipitation range, and it is during this period that the gaseousmedium is modifled to cause precipitation of the carbon. Thismodification consists of the admission of a deflnite amount of excessoxygen, either in the form -of atmospheric air or pure oxygen which willcombine with the inert atmosphere to produce aslightly oxidizingatmosphere. This may be accomplished by changing the conditions in the1,0

combustion chamber, or by adding oxygen to the products of combustionthrough a separate pipe, either in the form of atmospheric air, or ifdesired, pure oxygen. The volume of oxygen admitted to the heat treatingchamber is relatively small, for example, from 1% to 3%, although it maybe higher at times, depending upon the temperature of the material atthe time of admission, it being necessary at all times to guard againstthe admission of an amount of oxygen sufficient to cause a burning ofthe material treated. The addition of too much oxygen causes a suddenrise'in the temperature of the material which can be indicated by apyrometer in contact with the metal. The pyrometric reading serves as avisible warning when an excess of oxygen is being admitted. Preferably,admission of oxygen is continued throughout the entire carbonprecipitation range,

causing the precipitation of the carbon as temper carbon and theoxidization of a portion thereof. When the carbon precipitation issufficiently complete for the particular type of malleable iron desired,the heat is 'shut off, and the container allowed to cool in a sealedcondition. Thecooling usually takes from twelve to fourteen hours.

If it is desired to increase the surface ductility and resistance tocorrosion of the material treated, a substantially carburizing gaseousmedium, such as carbon monoxide, may be introduced into the heatingreceptacle. This may be produced by adding carbonaceous gas liquid orsolid material in the combustion chamber used to produce a substantiallyinert gaseous medium and the carbon monoxide gas thus generated isintroduced into the heating receptacle through the pipes provided, orcarbon monoxide may be otherwise introduced into the heating receptacle.This. carburizing ,medium is introduced subsequent to the carbonprecipitation range, and it may be desirable to raise the temperature inthe heating receptacle to ap,- proximately 1600 degrees F., or more. Theheat should be maintained at this high temperature from one to eighthours, depending upon the thickness of the skin or the degree ofpenetration desired.

, It will be understood that the cast iron referred to throughout thisspecification will be of a chemical analysis usually considered goodpractice in the production of malleable iron castings.

Considerable modification is possible in the lengths of time oftreatment, the temperatures employed and the amount of oxygen admitted,without departing from the essential features of my invention.Obviously, the thickness of the casting will have a bearing upon thetime necessary to accomplish the process and will govern the amount ofoxygen or oxidizing element used.

I claim:

1. The process of producing malleable iron which comprises placing castiron in a heating receptacle, displacing the, normal atmospheretherefrom by the passage of a substantially inert non-oxidizing gaseousmedium therethrough,

heating the cast iron in this inert non-oxidizing medium during thepreliminary heating and at an elevated temperature until substantiallyall the graphitic carbon in the cast iron has been absorbed by the iron,and thereafter reducing the temperature of the cast iron to allow carbonprecipitation to take place and admitting a relatively smallcontrollable percentage of an oxidizing gaseous medium to the heatingreceptacle during at least a portion of the carbon precipitation rangeto cause precipitation of the carbon as temper carbon and the oxidationof at least a portion thereof.

2. The process of producing malleable iron which comprises placing castiron in a heating receptacle, displacing the normal atmosphere therefromby the passage of a substantially inert non-oxidizing gaseous mediumtherethrough, heating the cast iron in this inert non-oxidizing gaseousmedium during the preliminary heating and at'a temperature betweenapproximately 1700 F. and 1800 F. until substantially all the graphiticcarbon in the cast iron has been absorbed by the iron, and thereaftergradually reducing the temperature to approximately 1200 F. to allowcarbon precipitation to take place and admitting a relatively smallpercentage of an oxidizing gaseous medium to the heating receptacleduring at least a portion of the carbon precipitation range to causeprecipitation of the carbon as temper carbon and the oxidation of atleast a portion thereof.

3. The process of producing malleable iron which comprises placing castiron in a heating receptacle, displacing the normal atmosphere therefromby the passage of a substantially inert non-oxidizing gaseous mediumtherethrough, heating the cast iron in this inert non-oxidizing mediumduring the preliminary heating and until substantially all the graphiticcarbon in the cast iron has been absorbed by the iron, thereafterreducing the temperature of the cast iron to allow carbon precipitationto take place and admitting a relatively small controllable percentageof an oxidizing gaseous medium to the heating receptacle during at leasta portion of the carbon precipitation range to cause precipitation ofthe carbon as temper carbon and the oxidization of at least a portionthereof, and thereafter bringing a carburizing gaseous medium intocontact with the cast iron at a carburizing temperature to increase thecarbon content of the surface of the cast iron.

4. The process of producing malleable iron which comprises placing castiron in a heating receptacle, displacing the normal atmosphere therefromby the passage of a substantially inert non-oxidizing gaseous mediumtherethrough,

heating the cast iron in this inert non-oxidizing gaseous medium duringthe preliminary heating and at a temperature between approximately 1700"F. and 1800%F. until substantially all the graphitic carbon the castiron has been absorbed by the iron, thereafter gradually reducing thetemperature to approximately 1200" F. to allow carbon precipitation totake place and admitting a relatively small percentage of an oxidizinggaseous medium to the heating receptacle during at least a portion ofthe carbon precipitation range to cause precipitation of the carbon astemper carbon and the oxidation of at least a portion thereof, andthereafter re-heating the cast iron to a carburizing temperature andbringing a' carburizing gaseous medium into contact with the cast ironat this temperature to increase the carbon content of the surface of thecast iron.

5. The process of producing malleable iron which comprises placing castiron in a closed heating receptacle, displacing the normal atmospheretherefrom by the passage of a substantially inert non-oxidizing gaseousmedium tberetbrough, continuously passing an inert non-oxidizing gaseousmedium through said heating receptacle during the preliminary heatingand at an elevated temperature until substant'i'ally all the graphiticcarbon in the cast iron has been absorbed by the iron, and thereafterreducing the temperature of the cast iron to allow carbon precipitationto take place while continuously passing a gaseous medium through saidheating receptacle, said gaseous medium containing a relatively smallcontrollable amount of oxygen during at least a portion of the carbonprecipitation range to cause precipitation of the carbon as tempercarbon and the oxidation of at least a portion thereof.

5. The process of producing malleable iron which comprises placing castiron inaclosedheating receptacle, displacing the normal atmospheretherefrom by the passage of a substantially inert non-oxidizing gaseousmedium therethrough, continuously passing an inert non-oxidizing least aportion thereof, and thereafter passing a carburizing gaseous mediumthrough said receptacle in contact with said iron maintained at acarburizing temperature to increase the carbon content of the surface ofthe cast iron.

WALTER CO'ITERILL.

